1985 300CD-T: Failing front brake caliper?

[5cylinder]

My vehicle:
1985 300CD-T (Calif.), W123.153

Issue:
Driving on the freeway, I smelled what I believed to be a "burning brake" odor. In attempting to slow down, I felt two bumps through the brake pedal as I was applying moderate pressure and the pedal was moving toward the floor. Stopped, I determined that my vehicle was the source of the odor. The odor appeared to be coming from the front left wheel.

I inspected under the hood and under the vehicle. No obvious signs of damage or leaking fluids either on the ground or vehicle. Fluid reservoirs appeared full.

With the front end of the vehicle off the ground, I attempt to manually spin the left wheel. It was far harder to spin than the right wheel.
I removed the front left wheel. No signs of anything amiss.

I removed the outer brake pad. All appeared okay. I carefully and slowly pressed the piston into the caliper as I would do as part of the job of replacing brake pads. All appeared okay.

I experienced great difficulty attempting to remove the inside brake pad. It seemed greatly hung up. Eventually, I was able to remove it. I carefully and slowly attempted to press the piston into the caliper. It would not (or barely) move.

I cleaned the caliper, re-installed both pads, and went for a drive. Once again, I felt two bumps through the brake pedal as I applied moderate pressure and the pedal was moving toward the floor. I am no longer smelling the odor.

As for the front brake calipers:
Bendix installed by an independent technician in May of 2000
88,246 miles on them

Question: Does this seem to be a matter of a failing caliper (as I believe)?

Thanks for any insight and advice with this situation.

[mannys9130]

Has your brake fluid been changed every 2 years?

[5cylinder]

Originally Posted by mannys9130

Has your brake fluid been changed every 2 years?

Yes.

[mannys9130]

Hmmm. That would seem to speak against failure. One piston is functional and the other sticks? That rules out brake hoses... I'd try to get that stuck piston mobile again and make sure that the pad and rotor face on the stuck side isn't cracked and glazed.

[5cylinder]

Originally Posted by mannys9130

Hmmm. That would seem to speak against failure. One piston is functional and the other sticks? That rules out brake hoses... I'd try to get that stuck piston mobile again and make sure that the pad and rotor face on the stuck side isn't cracked and glazed.

You're not the only one who would like to "...try to get that stuck piston mobile again".
On inspection, no flaws found on either pad or on the rotor. Pads and rotor have only 13,000 miles on them.
Thx.

[t walgamuth]

Does the car stop straight and true without pulling to one side?

[5cylinder]

Originally Posted by t walgamuth

Does the car stop straight and true without pulling to one side?

Seems to stop straight.

[mannys9130]

Do these two "bumps" happen every time you press the brake pedal?

[5cylinder]

Originally Posted by mannys9130

Do these two "bumps" happen every time you press the brake pedal?

The two bumps felt through the brake pedal happen when the engine is running and the vehicle is moving. They are not felt with light pressure. They are felt at what is medium-to-firm pressure on the pedal.

[Diesel911]

If a Caliper Piston is sticking you need to remove the Brake Caliper and remove the Piston to find out what is causing the issue.

Noraml Brake Fluid absorbs Water from the Air and that Water eventually gets down to the Calipers and causes rust/sludge and pitting inside of the Caliper.

Usually the Caliper bore is what takes the hit. Next the Pistons themselves can also pit and rust.

If you were to get a set of newely rebuilt Calipers and take it apart I bet you are going to find pits in the Caliper bore where past rust has eaten.

Another issue is that the elasticity of the square O-ring Caliper Seal is the only thing that pulls the Caliper Piston back. When it gets hard that won't happen or at lest it won't be the rubber that brings the Piston back.

You can buy Caliper Seal Kits and there is threads on how to install them.

[tyl604]

If you feel the wheel, I will bet it is hot. Sticking piston. Replace caliper. Happened on my 81 300SD; would drive around the block and the wheel would heat up. Eventually had to replace both rear calipers as the other one started sticking too soon thereafter. No problem in two to three years now.

[5cylinder]

Diesel911: Thanks for the very helpful and lengthy write-up! Very useful for improving my understanding. I appreciate it.

tyl604: The wheel did seem hot. Clearly a sticking piston.

Easy fix: I replaced both front Bendix calipers w/rebuilt Bendix calipers. Inexpensive fix, too: About $155 in parts. Quite reasonable considering, as I initially stated, almost 16 years old and 88,200+ miles on the replaced calipers. I had no interest in doing caliper rebuilding myself.

Thanks for all of the help on this thread!